Process for the electrolytic production of ammonium perphosphate in solid form



Patented Nov. 8, 1938 UNITED STATES PROCESS FOR THE ELECTROLYTIC PRO- DUCTION F AMltioNlUM PERPHOSPHATE IN SOLID FORM Walter Voelkel, FrankIort-on-the-Main, Germany, assignor to Deutsche Gold und Silber Scheideanstalt vormals Roessler, Frankfort-onthe-Main, Germany, a corporation of Germany No Drawing. Application July 3, 1935, Serial No.

8 Claims.

This invention is concerned witha process for preparing ammonium perphosphate electrolytically. As a further object of the invention the ammonium perphosphate salt, formed as a result of the electrolysis, is separated in solid form from the solution in which it is dissolved.

It is already known that solutions of ammonium. phosphates, especially those which contain secondary ammonium phosphate,

and tertia ry ammoniumphosphate (NI-IOaPOa can be converted electrolytically by anodic oxidation into'a solution containing ammonium per- 15 phosphate, ii the electrolysis is carried out in to electrolysis. Preferably the solutions of secondary ammonium phosphate which are electrolyzed are concentrated solutions. The resulting 30 electrolyte, which contains ammonium perphosphate', and the non-oxidizing salt, ammonium phosphate, may be readily treated in accordance with the process hereinafter more fully set forth to yield ammonium perphosphate in solid form.

Preferably a concentrated solution of secondary ammonium phosphate is electrolyzed in an electrolytic cell having no diaphragm. The anode may be formed of platinum and the oathode of an alloy steel such as V2A steel. The addition of substances for increasing the conductivity of the bath is not necessary. However, ii. desired, they may be added but they need be present only in very small amount. However the addition of a small amount of a chromate salt such as am- 45 monium chromate has been found desirable since these salts iunction as depolarizers. Current densities should not be too high and may be relatively low, and the temperature may preferably -be maintained somewhat below room tem- 0 ,perature.

After the electrolysis the electrolyte may be permitted to stand for 12 to hours in order to allow the ammonium monoperphosphate, NH4PO5, which is a undesirable by-product of 55 the electrolysis, to decompose with formation of gaseous oxygen and di-ammoniumphosphate. This decomposition takes place more readily when free ammonia is introduced into the solution. The solution is then treated to recover the 60 ammonium perphosphate. First' the solution In Germany July 9, 1934 containing the ammonium perphosphate and the secondary ammonium phosphate which has not been converted into the persalt is freed from ammonium phosphate and then the liquid is further treated'to obtain the perphosphate salt in solid form. Various methods of recovering the persalt are described below.

In'order to free the liquid from unconverted ammonium phosphate ammonia may-be added either in gaseous state or as solution in order to change the secondary ammonium phosphate into the difllcultly soluble tertiary ammonium phosphate, (NH4):P04, which is precipitated. The precipitation of the unconverted ammonium phosphate in this manner may be rendered substantially complete: If desired, the tertiary ammonium phosphate thus separated can be readily converted into secondary ammonium phosphate by boiling and used over again in the electrolytic process.

The ammonium perphosphate remaining in the solution may be recovered in solid form by the use of any one of various methods. As one method the solution may be. evaporated pref erably under a partial vacuum, and the desired perphosphate will remain behind. Or, an additional quantity of ammonia may be added to the solution 'of the persalt and the temperature lowered to below 0 C. Ammonium perphosphate in substantially quantitative yields is precipitated at this temperature.

As an alternative method for the recovery of ammonium perphosphate in solid form from solutions containing it, methyl alcohol or any other liquid miscible with water may be added to the solution. In this way the perphosphate salt is also precipitated. When using addition agents such as methyl alcohol, it is also desirable to free the electrolyte from the ammonium phosphate present therein by the addition of ammonia thus precipitating the diiiicultly soluble tertiary ammonium salt, before recovering the persalt.

As another possible method it desired, the electrolyte may be immediately evaporated to dryness, thus recovering a mixture of ammonium phosphate and ammonium perphosphate.

As an example 01' the improved process the following may be given:

Example 200 grams of secondary ammonium phosphate, (NH02HPO4, dissolved in 500 cc. of water were subjected to electrolysis for 6 to 7 hours. A current density of 0.05 ampere per square centimeter was used. The'applied potential was 4 volts. The electrolyte was cooled to a temperature of about 10 C.

The electrolyte was then permitted to stand for 12 to 15 hours and then treated to recover the ammonium perphosphate in dry form. Ammonia gas was first introduced into the electrolyte until no additional tertiary ammonium phosphate (NHQQPOi, was precipitated. About 20 or 30 grams of ammonia were introduced before the precipitation was complete. I

The precipitated ammonium phosphate was then filtered off and additional ammonia in gaseous form was introduced into the filtrate. During this step the temperature was maintained at about -5 C. Substantially pure ammonium perphosphate was precipitated and the yield was about The precipitated-ammonium perphosphate was removed by filtration and dried. The ammonia still remaining in the liquid was regained by familiar methods.

I claim:

1. A process for preparing substantially pure solid ammonium perphosphate which comprises electrolyzing a solution of secondary ammonium phosphate, addingamn'ionia to the resulting electrolyte in order to precipitate unchanged secondary ammonium phosphate therein, and .then treating said solution-in order to recover the desired ammonium perphosphate in the solid state.

2. A process for preparing substantially pure solid ammonium perphosphate which comprises electrolyzing a solution of secondary ammonium Ephosphate, precipitating unconverted ammonium phosphate from the electrolyte by addition of ammonia, freeing the solution from precipitated ammonium triphosplrate and evaporating the solution thus obtained to dryness. 3. A process for preparing substantially pure solid ammonium perphosphate which comprises electrolyzing a solution of secondary ammonium phosphate, precipitating unconverted ammonium phosphate from the electrolyte by addition of ammonia, freeing the solution from precipitated ammonium triphosphate and recovering from-the solution thus obtained the ammonium perphosphate in solid form.

4. A process for preparing ammonium perphosphate which comprises electrolyzing a solution of secondary ammonium, phosphate, precipitating,

unconverted ammonium phosphate from the electrolyteby addition oi 'ammonia, freeing the solution from precipitated ammonium triphosphate and precipitating from the solution thus obtained the ammonium perphosphate by adding further amounts of ammonia, said solution being maintained at a temperature below 0 C. during said precipitation. v

5. A process for preparing substantially pure solid ammonium perphosphate which comprises electrolyzing a solution of secondary ammonium phosphate, precipitating unconverted ammonium phosphate from the electrolyte by addition of ammonia, freeing the solution from precipitated ammonium triphosphate and precipitating from the solution thus obtained the ammonia perphosphate by adding an organic liquid miscible with water.

6. A process for the preparation of substantially pure solid ammonium perphosphate which comprises electrolyzing a concentrated solution of secondary ammonium phosphate, permitting the resulting electrolyte to stand until the ammonium monoperphosphate contained therein is substantially decomposed, introducing ammonia in order to precipitate tertiary ammonium phosphate, filtering to remove the precipitated tertiary ammonium phosphate and then recovering the ammonium perphosphate in substantially dry form from the filtrate.

7. A process for the preparation or ammoniumperphosphate which comprises electrolyzing a concentrated solution of secondary ammonium phosphate, introducing ammonia in gaseous form into the electrolyte in order to precipitate tertiary ammonium phosphate, removing the precipitated ammounium phosphate by filtration, and introducing additional ammonia into the filtrate and cooling said filtrate to a temperature of 0 C. or below in order to precipitateammonium perphosphate in solid form.'

8. A process for the preparation of substantially adding methyl alcohol to the filtrate in order to precipitate ammonium perphosphate in solid form.

WALTER vonrm. so 

